Methods
Trial Design
We used sequential randomization to individually
evaluate the tracking and texting components of the
intervention. After establishing baseline activity during a
blinded run-in (week 1), in phase I (weeks 2 to 3) we
randomized 2:1 to unblinded versus blinded tracking. The
activity tracker itself did not show activity information, but
continuously transmitted it in all participants. The activity data
were only visible to those who were unblinded, as further
described below. In phase II (weeks 4 to 5), we randomized
unblinded participants 1:1 to“smart texts” versus no texts.
Smart texts were automated, personalized, smartphonedelivered coaching messages informed by real-time activity
and other factors